Especially with the advent of high speed xerographic reproduction machines wherein copiers or printers can produce at a rate in excess of one hundred and twenty pages per minute (PPM), there is a need for sheet handling systems to feed paper or other substrate through each process station in a rapid succession in a reliable and dependable manner in order to utilize the full capabilities of the reproduction machine. These sheet handling systems must operate flawlessly to virtually eliminate the risk of damaging the substrate and to minimize machine shutdowns due to misfeeds or multifeeds. It is in the initial separation of the individual sheets from the substrate stack where the greatest number of problems occur.
One of the sheet feeders best known for high speed operation is the top vacuum corrugation feeder with front air knife. In this system, a vacuum plenum with a plurality of friction belts arranged to run over the vacuum plenum is placed at the top of a stack of sheets in a supply tray. Several fluffers are located around the perimeter of the stack for injecting air into the top of the stack. When vacuum is supplied to the vacuum plenum, the resulting vacuum field draws one or more sheets against the friction belts. At the front of the stack, an air knife is used to inject air into the acquired sheets to separate the top sheet from the remainder of the sheets which then are pushed down onto the stack. In operation, the vacuum pulls one or more sheets up and acquires them, and then air is injected by the air knife toward the acquired sheets to separate the top sheet. Following separation, the belt transport drives the sheet forward off the stack of sheets. In this configuration, separation of the next sheet cannot take place until the top sheet has cleared the stack. In this type of feeding system every operation takes place in succession or serially and therefore the feeding of subsequent sheets cannot be started until the feeding of the previous sheet has been completed.
A variation of the paper feeder technology described above uses a reciprocating feedhead in lieu of a friction belt transport to drive the top sheet into the paper path, e.g., U.S. Pat. No. 6,264,188. At the appropriate time during the feed cycle, the feedhead moves towards take away rolls, carrying the acquired top sheet with it. The leading edge of the top sheet then enters the take away roll nip, and the take away rolls remove the sheet from the feedhead, which then cycles back to its original position. Within the feedhead are several parallel ribs which induce a corrugation pattern in the acquired sheets, thus creating gaps between the sheets, facilitating sheet separation by the air knife
Current top and bottom vacuum corrugation feeders utilize a valved vacuum feedhead, e.g., U.S. Pat. No. 4,269,406. At the appropriate time during the feed cycle the valve is actuated, establishing a flow and hence a negative pressure field over the stack top or bottom if a bottom vacuum corrugation feeder is employed. This field causes the movement of the top sheet(s) to the vacuum feedhead where the sheet is then transported to the take away rolls. Once the sheet feed edge is under control of the take away rolls, the vacuum is shut off. The trail edge of this sheet exiting the feedhead area is the criteria for again activating the vacuum valve for the next feeding.
A method for measuring substrate bending stiffness and thereby basis weight on a real time basis is provided in the disclosed embodiment. A corrugator having a plurality of ribs is provided, with one or more sheets of the substrate provided below the corrugator wherein a predetermined gap exists between a topmost sheet of the sheets and the corrugator. A vacuum is applied between the corrugator and the topmost sheet wherein the vacuum is sufficiently large to raise the topmost sheet thereby deflecting and bending it into a profile corresponding to the arrangement and size of the corrugator ribs and bending stiffness of the substrate. One or more sensors may be provided for measuring the deflection of the topmost sheet. The vacuum, an air knife output and/or a fluffer output may then be adjusted according to predetermined rules and the measured deflection.
FIG. 1 is one example of a feedhead corrugator and two sheets of substrate prior to application of a vacuum;
FIG. 2 is the feedhead corrugator and two sheets of 300 gsm paper subsequent to application of a vacuum;
FIG. 3 is the feedhead corrugator and two sheets of 110# paper subsequent to application of a vacuum;
FIG. 4 is the feedhead corrugator and two sheets of 32# paper subsequent to application of a vacuum;
FIG. 5 is the feedhead corrugator and two sheets of 20# paper subsequent to application of a vacuum;
FIG. 6 is the feedhead corrugator and two sheets of 13# paper subsequent to application of a vacuum;
FIG. 7 is a distribution of sensor output voltages for various paper basis weights for a number of runs;
FIG. 8 is an alternate feedhead corrugator and two sheets of 75 gsm (20# bond) paper subsequent to application of a vacuum;
FIG. 9 is a graph of bending profiles for papers of a variety of basis weights;
FIG. 10 is a graph of substrate deflection versus substrate basis weight; and
FIG. 11 is a representational schematic of a reprographic system according to the present invention.